Our invention is in the general area of orthopedic prostheses, and in particular tibial prostheses.
The tibia is situated at the front and inner side of the leg and, except for the femur, is the longest and largest bone in the skeleton. It is prismoid in form, expanded above, where it enters into the knee joint. The head of the tibia is large and is expanded on each side into two eminences, the tuberosities. These present two smooth concave surfaces which articulate with the condyles of the femur. The medial condyle is more prominent anteriorly and broader both in the anterior-posterior and transverse diameters than the lateral condyle. Accordingly, the lateral articular surface of the tibia is longer, deeper and narrower than the medial surface of the tibia, so as to articulate with the lateral condyle. The medial surface is broader and more circular, concave from side to side, to articulate with the medial condyle. The anterior surfaces of the tuberosities are continuous with one another, forming a single large surface which is somewhat flattened. Posteriorly the tuberosities are separated from each other by a shallow depression for attachment of ligaments. The inner tuberosity presents posteriorly a deep transverse groove for the insertion of a tendon.
Through aging or disease, the articulating surfaces of the femur and the tibia may degrade, and replacement with the prostheses may be necessary. Numerous designs for prostheses have been proposed, but none has gained universal acceptance. There remains room for improvement, particularly because of the interaction between biological structures and a prosthesis.
For example, when a tibial prosthesis is implanted, it is a common practice to resect the head of the tibia. The surgeon usually removes the tuberosities by sawing across the head of the tibia with a sagittal saw. This produces a relatively flat surface, preferably perpendicular to a mechanical axis of the tibia so that the forces produced in the knee joint will be generally uniformly distributed. The bone of the tibia, however, is not uniform in character. The outer surface is less porous than the inner and is called cortical bone. The more porous inner portion is called cancellous bone. The cancellous bone is somewhat spongy and forms a lattice work within the cortical bone. At the head of the tibia, the cortical bone has both an irregular upper surface and a varying thickness. When the tibial head is resected, the structural characteristics of the upper surface of the tibia are altered. In some places, the cortical bone may be completely removed, and the inner cancellous bone exposed. The thickness and strength of the cortical bone will be altered. Because of the variation between individuals, as well as the effects disease and aging, the strength of a particular resected surface is unpredictable. The structural characteristics of the tibia supporting the prosthesis affect the long-term viability of the prosthesis.
Traditionally, tibial prostheses have comprised a rigid metal baseplate to be affixed to the resected upper surface of the tibia and an ultra-high molecular weight polyethylene upper part with articulating surface which attaches to the baseplate. We have found that such prostheses can loosen over time. It is not unusual for the bony structures under the tibial baseplate to respond differently to the loads applied by a patient during walking or other movement. For example, if more of the compact bone had been removed from the medial side of the tibia during resection, the bone under the medial side of the baseplate would be more likely to compress. Over time, the planer resected surface created by the surgeon for the implantation of the prosthesis may deform into a curved surface. Under continued use, the rigid baseplate would tend to rock on the curved surface, ultimately affecting whatever means had been chosen to attach the prosthesis to the tibia. Eventually, the fixation means may fail, in whole or in part, and new surgery may be required.
This problem has been recognized in the past, and some solutions have been proposed which ameliorate the effects described. One solution is to provide separate structures for the medial and lateral condyles. Such a solution is shown, for example, in U.S. Pat. Nos. 3,774,244 to Walker, or 4,034,418 to Jackson or 4,085,466 to Goodfellow. To implant these structures, however, requires that four cooperating structures be implanted, two on the tibia and two on the femur. The possibility of an error in implantation is increased over a unitary construction.
Another partial solution is inherent in U.S. Pat. No. 4,711,639 to Grundei. Grundei proposed a unitary baseplate with two separate articulating surfaces. A small amount of play in the attachment between the articulating surfaces and the baseplate would permit each surface to flex up and down slightly. We find, however, that this solution is inadequate because the tibial prosthesis is only loaded in compression provisioned for separate articulating surfaces, therefore, does not change the action of the baseplate and degradation of the attachment means is still possible.
U.S. Pat. No. 4,883,488 to Bloebaum, et al., reduces the difficulties of implantation by connecting two baseplates together with a slide key. A separate articulating surface is provided for each baseplate.
Another proposed solution is represented by U.S. Pat. No. 4,673,407 to Martin. Martin discloses a prosthesis fixation apparatus comprising a low modulus spring interposed between a cancellous bone screw and a baseplate. This seeks to reduce the affect of rocking on the fixation means, but it does not eliminate or reduce rocking of the baseplate to a significant extent. Bone ingrowth into such a prosthesis would be impeded by the rocking action.